1. Field of the Invention
This invention generally relates to alkali-doped metal chalcogenides and, more particularly, to a processes for forming sodium-doped metal chalcogenides using solutions of metal precursors.
2. Description of the Related Art
Metal and mixed-metal chalcogenides represent important classes of semiconductor materials for electronic and photovoltaic (PV) applications. In particular, copper indium gallium diselenide (CuIn1-xGaxSe2 or CIGS) has emerged as a promising alternative to other existing thin-film technologies. Overall, CIGS exhibits a direct and tunable energy band gap, high optical absorption coefficient in the visible to near-infrared (NIR) spectrum and has demonstrated power conversion efficiencies (PCEs)>20%. Conventional CIGS fabrication (vacuum) processes typically involve either sequential or co-evaporation (or sputtering) of copper (Cu), indium (In), and gallium (Ga) metal onto a substrate followed by annealing in an atmosphere containing a selenium source to provide the final CIGS absorber layer.
In contrast to vacuum approaches, which create an environment to control variables such as the gases introduced and pressure, non-vacuum methods offer significant advantages in terms of both reduced cost and high throughput manufacturing capability. Electrodeposition or electroplating of metals (from metal ions dissolved in solution) onto conductive substrates represents an alternative CIGS fabrication strategy. Finally, CIGS fabrication via deposition of mixed binary, ternary, and/or quaternary nanoparticles of copper, indium, gallium, and selenium (nanoparticle “inks”) embodies another non-vacuum approach.
In addition to the approaches described above, a number of alternative approaches and hybrid strategies have been reported with varying degrees of success. Overall, CIGS fabrication via solution-based approaches appears to offer a convenient, low-cost option. According to this method, metal salts or metal complexes (precursors) of copper, indium, and gallium are dissolved in a solvent to form a CIGS ink and subsequently deposited on a substrate to form a film using conventional methods.
Mitzi et al. described a solution-based CIGS absorber layer deposition strategy using homogenous solutions of Cu, In, Ga and Se (and optionally sulfur) obtained by dissolution in hydrazine without the requirement for post-deposition, high-temperature selenization.1-3 Subsequently, a hydrazine-free approach was reported whereby isolated hydrazinium-based precursors could be deposited to form metal chalcogenide composite films.4 More recently, Todorov et al. described a 15.2% PCE for a thin-film solar cell with a solution-processed Cu(In,Ga)(S,Se)2 (CIGS) absorber, which represents the highest reported value for a pure solution deposition method.5 With the exception of the aforementioned cases, solution-based deposition methods for fabricating a CIGS absorber layer have historically demonstrated significantly lower performance compared to vacuum and/or electrodeposition processes.
Keszler et al. described a solution-based approach for the fabrication of low contamination metal chalcogenides in aqueous media.6 In general, the formulation consists of aqueous metal chalcogenide precursors as a mixture of metal cation salts, formate anions, and a source of chalcogen (selenium, sulfur) in the form of thermally labile precursors including thiourea, thioformamide, selenourea, selenoformamide, etc. Overall, this method offers both environmentally favorable processing and low CIGS film contamination due to the careful selection of appropriate precursor materials. Finally, Wang et al. reported an inkjet printing method whereby the CIGS precursor film was printed on molybdenum (Mo)-coated substrates from a solution of Cu, In and Ga sources containing ethylene glycol and ethanolamine.7 Following thermal selenization and subsequent CIGS device integration, a PCE of 5.04% was obtained. Subsequently, Wang et al. demonstrated a PCE exceeding 8% for CIGS solar cells through careful optimization of Cu, In, and Ga precursor formulations.8 
As previously mentioned, solution processing methods for CIGS generally suffer from lower performance relative to conventional vacuum and electrodeposition approaches, with the exception of the aforementioned hydrazine-based method. In response to this, appropriate doping strategies offer the potential to dramatically improve the morphology and PV behavior of the CIGS absorber, thereby decreasing the performance “gap” between solution and vacuum processes. Fortunately, CIGS exhibits a robust tolerance towards defects and/or impurities. In light of this, benefits from doping have been successfully exploited through improvements in CIGS layer morphology (grain size) that ultimately translate into better device performance primarily through a reduction of grain boundaries, which effectively suppresses recombination phenomena.
One conventional strategy for improving the performance of CIGS solar cells involves using soda-lime glass (SLG) substrates, through which Na migration during thermal treatment affords a Na-doped CIGS absorber layer. In the case of SLG, Na diffusion proceeds through the Mo back contact which further necessitates control over Mo properties. Indeed, a number of explanations and models have been proposed to rationalize the beneficial impact of Na-doping on CIGS solar cell performance.9,10 However, the dominant phenomena appear to manifest themselves in terms of improvements in (1) output voltage, (2) CIGS grain morphology or, in some cases, both (1) and (2).
In addition to the straightforward exploitation of Na migration from SLG during thermal treatment as a method for realizing a Na-containing CIGS absorber layer (passive approach), alternative (active) strategies for Na-doping of CIGS have been reported. Ård et al. described CIGS growth on Mo-substrates containing layers of NaF deposited prior to CIGS deposition.11 For this study, the substrates consisted of SLG both with and without Na-diffusion barriers. With respect to CIGS solar cell devices, improvements in grain structure, film texture, and surface flatness were observed when NaF precursor layers were employed relative to both Na-free and Na-poor samples. In terms of device performance, solar cell efficiency was shown to increase in response to modest additions of Na. Yun et al. demonstrated improved photovoltaic properties for CIGS solar cells fabricated on a Na-free substrate (alumina) by employing Na-doped Mo as the bottom layer of a Mo back contact.12 According to this method. Na-doped Mo was shown to function as a Na source material wherein control over Na content was realized by tuning the thickness of the Na-doped Mo layer without the need for an alkali diffusion barrier. Shin et al. employed a Na-doping strategy by incorporating an Na2S precursor at three distinct points in a three-stage co-evaporation process for CIGS fabrication.13 However, it was determined that the performance of CIGS solar cells employing external Na-doping of CIGS fabricated on Mo/Corning glass substrates was worse than devices prepared using conventional methods. Mackie et al. provided a method for alkali doping of PV materials whereby the alkali-containing transition metal layer was fabricated by sputtering from a first target comprising a transition metal and a second target comprising an alkali metal.14 
Not surprisingly, the high performance demonstrated for CIGS solar cells fabricated by conventional (vacuum or other) processes as compared to solution-based methods have dominated the research and development landscape until only recently. Nevertheless, a few examples have been previously provided that warrant discussion. Basol et al. described a method for fabricating compound semiconductor films using a source material, which may be provided in the form of an ink from particles in a powder form.15 Furthermore, the source material includes Group IB-IIIA alloy containing particles containing at least one Group IB-IIIA alloy phase. Subsequently, a precursor film prepared from the source material is thermally treated in an appropriate atmosphere to furnish a composite film comprising a Group IB-IIIA-VIA compound. Finally, Kapur et al. provided a method for fabricating a compound film that involves the preparation and subsequent deposition of a source material to form a preparatory film, thermally treating the preparatory film in an appropriate atmosphere to form a precursor film which further includes providing material to the precursor film to form the compound film.16 In this case, the source material comprises oxide-containing particles including Group IB and IIIA elements, while the precursor film includes non-oxide Group IB and IIIA elements. Finally, the compound film includes a Group IB-IIIA-VIA compound and optionally may contain a dopant.
1. D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, V. Deline and A. G. Schrott, “A High-Efficiency Solution-Deposited Thin-Film Photovoltaic Device”, Advanced Materials 2008, 20, 3657-3662.
2. D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, L. Gignac and A. G. Schrott, “Hydrazine-Based Deposition Route for Device-Quality CIGS Films”, Thin Solid Films 2009, 517, 2158-2162.
3. D. B. Mitzi, W. Liu and M. Yuan, “Photovoltaic Device with Solution-Processed Chalcogenide Absorber Layer”, US2009/0145482 A1.
4. D. B. Mitzi and M. W. Copel, “Hydrazine-Free Solution Deposition of Chalcogenide Films”, US8, 134, 150 B2.
5. T. K. Todorov, O. Gunawan, T. Gokmen and D. B. Mitzi, “Solution-Processed Cu(In,Ga)(S,Se)2 Absorber Yielding a 15.2% Efficient Solar Cell”, Progress in Photovoltaics: Research and Applications 2012, doi:10.1002/pip.1253
6. D. A. Keszler and B. L. Clark, “Metal Chalcogenide Aqueous Precursors and Processes to Form Metal Chalcogenide Films”, US2011/0206599 A1
7. W. Wang, Y-W. Su and C-H. Chang, “Inkjet Printed Chalcopyrite CuInxGa1-xSe2 Thin Film Solar Cells”, Solar Energy Materials & Solar Cells 2011, 95, 2616-2620.
8. W. Wang, S-Y. Han, S-J. Sung, D-H. Kim and C-H. Chang, “8.01% CuInGaSe2 Solar Cells Fabricated by Air-Stable Low-Cost Inks”, Physical Chemistry Chemical Physics 2012, 14, 11154-11159.
9. U. P. Singh and S. P. Patra, “Progress in Polycrystalline Thin-Film Cu(In,Ga)Se2 Solar cells”, International Journal of Photoenergy, Volume 2010, Article ID 468147.
10. Y. Jeong, C-W. Kim, D-W. Park, S. C. Jung, J. Lee and H-S. Shim, “Field Modulation in Na-Incorporated Cu(In,Ga)Se2 (CIGS) Polycrystalline Films Influenced by Alloy-Hardening and Pair-Annihilation Probabilities”, Nanoscale Research Letters 2011, 6:581.
11. M. B. Ård, K. Granath and L. Stolt, “Growth of Cu(In,Ga)Se2 Thin Films by Coevaporation Using Alkaline Precursors”, Thin Solid Films 2000, 361-362, 9-16.
12. J. H. Yun, K. H. Kim, M. S. Kim, B. T. Ahn, S. J. Ahn, J. C. Lee and K. H. Yoon, “Fabrication of CIGS Solar Cells with a Na-doped Mo Layer on a Na-Free Substrate”, Thin Solid Films 2007, 515, 5876-5879.
13. Y. M. Shin, D. H. Shin, “Effect of Na Doping Using Na2S on the Structure and Photovoltaic Properties of CIGS Solar Cells”, Current Applied Physics 2011, 11, S59-S64.
14. N. M. Mackie, D. R. Julino and R. B. Zubeck, “Method for Alkali Doping of Thin Film Photovoltaic Materials”, U.S. Pat. No. 7,897,020 B2.
15. B. M. Basol, V. K. Kapur, A. T. Halani, C. R. Leidholm and R. A. Roe, “Method of Making Compound Semiconductor Films and Making Related Electronic Devices”, U.S. Pat. No. 5,985,691.
16. V. K. Kapur, B. M. Basol, C. R. Leidholm and R. A. Roe, “Oxide-Based Method of Making Compound Semiconductor Films and Making Related Electronic Devices”, U.S. Pat. No. 6,127,202.
It would be advantageous if a solution-based process existed for the fabrication of an alkali-doped metal chalcogenide, such as a sodium-doped Cu—In—Ga—Se (CIGS) composite material.